This brand-new cover art for the U.K might just give us a few clues on the plot of The Mark of Athena

What I Found

In the background you can see a longs ships sail, (The ArgoII) I think this symbolises a new quest, in which they will have to sail to some new destination.

In the front you can see Percy and Jason clashing swords in what seems like a battle, my guess is that there is some sort of dispute between the two camps, which would probably means the "Greek-Roman meet and greet" didn't go so well.